1. Field of the Invention
The present invention relates to a cooling fan for cooling a magnetron and a high voltage transformer disposed in a electric device chamber of a microwave oven, and more particularly, the present invention relates to a cooling fan for a microwave oven, which comprises a mixed flow fan having a specified hub angle, or an axial flow fan having optimized design parameters such as a ratio of an outer diameter of the fan to a width of a electric device chamber defined in a cabinet, a hub ratio, a sweep angle, a pitch angle, a maximum camber rate, etc.
2. Description of the Related Art
Generally, a microwave oven serves as an electric home appliance which is operated to induce increased molecular motion of water contained in a food item using high-frequency electromagnetic waves, cause molecules of water to vibrate, and generate heat within the food item to thereby cook the food item in a short period of time.
A space defined in a cabinet of a microwave oven is divided into a cooking chamber in which a food item is cooked, and a electric device chamber in which various electric devices are disposed.
FIG. 1 is a side cross-sectional view illustrating a state wherein a conventional cooling fan is disposed in a electric device chamber of a microwave oven.
As shown in FIG. 1, in the electric device chamber of the microwave oven, there are disposed up and down a magnetron 12 for radiating high frequency waves into a cooking chamber (not shown) defined in a cabinet 2 and a high voltage transformer 14 for applying a high voltage to the magnetron 12. A cooling fan 20 is arranged behind the magnetron 12 and high voltage transformer 14 to supply airflow and cool them. The cooling fan 20 is driven by a motor 16.
Conventionally, the cooling fan 20 comprises an axial flow fan which sucks air through air suction holes 2a defined in a rear wall of the cabinet 2 and discharges the air in an axial direction. The axial flow fan 20 has a hub 22 which is coupled to an output shaft 16a of the motor 16 to be integrally rotated therewith and a plurality of blades 24 which are installed on a circumferential outer surface of the hub 22 to be spaced apart one from another by a predetermined angle.
The axial flow fan 20 is arranged behind the magnetron 12 and high voltage transformer 14 which are disposed up and down in the electric device chamber. Concretely speaking, in order to ensure that airflow is evenly distributed over the magnetron 12 and high voltage transformer 14, the axial flow fan 20 is located between the magnetron 12 and high voltage transformer 14 along a vertical direction.
However, the conventional microwave oven constructed as mentioned above suffers from defects in that, since the magnetron 12 and high voltage transformer 14 are disposed up and down in the electric device chamber and the axial flow fan 20 is arranged behind them, a some portion of the airflow discharged at a high velocity from the axial flow fan 20 simply passes through a space which is defined between a lower end of the magnetron 12 and an upper end of the high voltage transformer 14. As a consequence, a cooling efficiency of the cooling fan 20 cannot but be deteriorated.
FIG. 2 is a perspective view independently illustrating the conventional cooling fan 20 of FIG. 1, and FIG. 3 is a partial side view illustrating a distal end of the blade 24 which forms a part of the cooling fan 20 shown in FIG. 1.
In the conventional axial flow fan 20, a ratio between an outer diameter of the fan 20 and a width of the electric device chamber defined in the cabinet 2, as measured on a z-axis of FIG. 1, is 0.74, a hub ratio between outer diameters of the hub 22 and the axial flow fan 20 is 0.23, a sweep angle is 0˜32°, and a pitch angle is 31˜45°.
Here, the pitch angle denotes an angle which is defined between a straight line connecting a leading edge LE to a trailing edge TE of the blade 24 and a line diametrically extending through the hub 22. Therefore, the pitch angle indicates a degree to which the blade 24 is inclined with respect to a plane perpendicular to the output shaft 16a of the motor 16.
Nevertheless, the conventional axial flow fan 20 configured as described above has disadvantages in that, when the axial flow fan 20 is driven by the motor 16 to be rotated at 2856 RPM, a volume flow rate sucked into the cabinet 2 is 1.73 CMM, and under this condition, a noise level reaches 43.1 dB[A], whereby considerable air suction noise is generated during rotation of the blades 24.
Also, when considering the fact that, as shown in FIG. 3, each blade 24 has a positive-pressure acting surface 24b on which a positive pressure is applied due to air suction and a negative-pressure acting surface 24c on which a negative pressure is applied due to air discharge, and a blade tip 24a formed between distal ends of the positive-pressure acting surface 24b and negative-pressure acting surface 24c has a slightly curved cross-section, since a static pressure regaining phenomenon quickly occurs in the air flowing from the positive-pressure acting surface 24b over the blade tip 24a toward the negative-pressure acting surface 24c, a noise level is further increased.